A linear solenoid that outputs a thrust force when magnetic flux is generated by energizing a coil has been conventionally known. Such a linear solenoid is installed in a vehicle, for example.
In order to expand a movement amount (hereinafter referred as a “stroke”) of a movable element in an axial direction without an increase in the size of the linear solenoid in the axial direction, a linear solenoid having a cylindrical portion, the movable element and a plurality of stators that are respectively disposed outside and inside of the cylindrical portion has been presented.
For example, a patent document (JP 2005-045217 A) discloses a linear solenoid including a magnetic movable element and a first to third stators.
The movable element includes a cylindrical magnetic portion and is coaxially housed inside a coil and movable in an axial direction of the coil. The first stator is disposed inside of the cylindrical magnetic portion of the movable element and allows the magnetic flux to flow toward the movable element in a radial direction (i.e., provides the magnetic flux to the movable element). The second stator is made of magnetic material and has a cylindrical shape. The second stator is disposed radially outside of the movable element and the movable element is interposed between the first stator and the second stator. The second stator also allows the magnetic flux to flow toward the movable element in the radial direction. Further, the third stator element is positioned away from the second stator element in the axial direction and attracts the movable element in the axial direction.
In a use of a linear solenoid, oil or water, etc. may be introduced into and discharged from the linear solenoid according to movement of a movable element. In such a use of the linear solenoid, oil or water, etc. needs to be smoothly introduced into and discharged from the linear solenoid in order to reduce energy loss due to the movement of the movable element. However, since the linear solenoid in the patent document has no configuration for the smooth introduction and discharge of oil or water, etc., the responsiveness of the movable element may be significantly deteriorated.
Furthermore, in the linear solenoid of the patent document, the cylindrical magnetic portion of the movable element has substantially the same length in the axial direction as that of the first stator element. Thus, when the stroke of the movable element is increased, areas of the movable element and the first stator element, which contribute (i.e., involve) to provide magnetic flux, are reduced, and thus attraction force may be reduced.